Our long-term goal is to continue the development of computational methods to describe quantitatively, at the molecular level, the physical forces responsible for the formation of macromolecular protein/lipid complexes at membrane surfaces. These complexes function in a variety of cellular processes such as interfacial signal transduction. Our overall hypothesis for the current proposal is that electrostatics plays a general role in phosphoinositide signaling and is manifested through phosphoinositide-mediated membrane association and the lateral organization of proteins and lipids. The first Specific Aim is to predict the membrane-associated orientations of phosphoinositide-binding domains and to describe quantitatively the effect of bound ligand on the electrostatic properties of these domains. Describing how phosphoinositides change the electrostatic character of these domains is crucial to understanding how membrane association is regulated: We hypothesize that binding of the highly negatively charged phosphoinositide neutralizes basic groups in the ligand-binding pocket and reduces the energetic barrier to insertion of hydrophobic motifs into the membrane interface. The second Specific Aim is to predict how phosphoinositides affect the membrane- associated orientations of C2 domains and, thus, contribute to calcium-mediated cellular processes such as synaptic vesicle exocytosis. We hypothesize that these interactions are driven mainly by non-specific electrostatic forces. The third Specific Aim is to simulate the lateral interactions among proteins and phosphoinositides at membrane surfaces in order to characterize the multimeric structures that form upon their organization. Such complexes play key roles in intracellular calcium release and cellular chemotaxis. We hypothesize that localized regions of negative and positive potential at the membrane surface serve as basins of attraction for molecules with opposite electrostatic characteristics and that electrostatics is a major driving force in forming these complexes. Our calculations will provide experimentally testable hypotheses for the membrane association of phosphoinositide binding domains, for membrane-mediated interactions, and for the regulation of both. A full description of electrostatic interactions will pave the way for the future study of other important interactions. Mutations that affect membrane targeting are implicated in cancer and microbial susceptibility. Thus, this research will provide insight into both normal and aberrant functionality.